Full moons vary in how large they appear because of the moon’s elliptical orbit. One side of the orbit – the perigee – is closest to the earth, while the other – the apogee, is the farthest away, with the difference being about 50,000 km, NASA reported.

On Saturday night, the moon will be only one hour away from perigee – or almost as close to the earth as it can get, NASA said. This only happens about once every 18 years, the U.S. Naval Observatory said in the NASA article.

The full moon will also bring perigean tides. But despite what you may hear from astrologers, actual scientists say this isn’t a big deal. The tide waters will only be pulled about 1 inch higher than usual – and up to 6 inches depending on local geography, NASA reported.

And besides that, while tides on ocean coastlines can range several feet, in Lake Michigan, the tide range is only a few inches anyway.

NASA says the best time to gaze into the night sky is when the moon is close to the horizon. For reasons not yet known by scientific experts, low moons appear especially large to the human eye when they are beaming through trees and buildings in the foreground, NASA said.

The forecast in Chicago for Saturday night is partly cloudy, with a 20 percent chance of showers after midnight and lows in the mid-30s, so it sounds like seeing the perigee moon is a good possibility.